In 2017, we added namespaced aliases so you could use namespaces to import
Twig classes, but we also maintained the non-namespaced classes, to not break
any existing applications. This is how the previous example looks like when
using namespaces:

Maintaining both ways of working with Twig classes complicates its maintenance
unnecessarily, so we've decided that it's time to switch all Twig classes to
use PHP namespaces. That's why, starting from the next 2.x version, all non-
namespaced classes will be deprecated and they will be removed in Twig 3.x,
which is expected to be released during 2019.

This move implies lots of code changes, so please test the development version
of Twig 2.x in your real applications and report any issues before we tag
its next stable version at the end of next week.